The Effect of Ambient Light Conditions on Quantitative Pupillometry

Automated devices collecting quantitative measurements of pupil size and reactivity are increasingly used for critically ill patients with neurological disease. However, there are limited data on the effect of ambient light conditions on pupil metrics in these patients. To address this issue, we tested the range of pupil reactivity in healthy volunteers and critically ill patients in both bright and dark conditions.

We measured quantitative pupil size and reactivity in seven healthy volunteers and seven critically ill patients with the Neuroptics-200 pupillometer in both bright and dark ambient lighting conditions. Bright conditions were created by overhead LED lighting in a room with ample natural light. Dark conditions consisted of a windowless room with no overhead light source. The primary outcome was the Neurological Pupil Index (NPi), a composite metric ranging from 0 to 5 in which > 3 is considered normal. Secondary outcomes included resting and constricted pupil size, change in pupil size, constriction velocity, dilation velocity, and latency. Results were analyzed with multi-level linear regression to account for both inter- and intra-subject variability.

Fourteen subjects underwent ten pupil readings each in bright and dark conditions, yielding 280 total measurements. In healthy subjects, median NPi in bright and dark conditions was 4.2 and 4.3, respectively. In critically ill subjects, median NPi was 2.85 and 3.3, respectively. Multi-level linear regression demonstrated significant differences in pupil size, pupil size change, constriction velocity, and dilation velocity in various light levels in healthy patients, but not NPi. In the critically ill, NPi and pupil size change were significantly affected.

Ambient light levels impact pupil parameters in both healthy and critically ill subjects. Changes in NPi under different light conditions are small and more consistent in healthy subjects, but significantly differ in the critically ill. Practitioners should standardize lighting conditions to maximize measurement reliability.